Electrical feedthroughs provide an electrical circuit path extending from the interior of a hermetically sealed housing of an implantable medical device (IMD) to the exterior of the housing. IMDs, such as cardiac pacemakers, implantable cardiovertor defibrillators, neuromuscular stimulators, and physiological monitors, employ such electrical feedthroughs to make electrical connection with leads, electrodes or sensors located outside the IMD housing. A conductive path is provided through the feedthrough by a conductive feedthrough pin which is electrically insulated from the IMD housing. IMDs commonly operate in association with multiple leads, electrodes or sensors and thus feedthrough arrays including multiple feedthroughs have been developed.
IMDs can be susceptible to electromagnetic interference (EMI), which can interfere with proper IMD function. As such, capacitive filter arrays have been incorporated in the feedthrough arrays to filter each of the feedthroughs, shunting EMI at the entrance to the IMD. Examples of capacitive filtered feedthroughs for use with an IMD are generally disclosed in commonly assigned U.S. Pat. No. 5,870,272 (Seifried et al.) and U.S. Pat. No. 6,414,835 (Wolf, et al.), both of which patents are incorporated herein by reference in their entirety.
IMDs are typically provided as programmable devices having RF telemetry circuitry adapted for bidirectional communication with an external programmer or monitor. Telemetry transmission systems commonly used with IMDs have typically relied upon the generation of low amplitude magnetic fields. Current oscillating in an LC circuit of an RF telemetry antenna in a transmitting mode induces currents in a closely spaced RF telemetry antenna in a receiving mode. An RF carrier frequency in some IMD products is set at a relatively low frequency of 175 kHz. The external RF telemetry antenna and the IMD RF telemetry antenna, which is typically enclosed within the IMD housing, are brought into close proximity with the use of a programming head.
Use of a telemetry system that requires a programming head limits the retrieval and transmission of data from/to an IMD to times when the patient can be positioned close to the programmer and the patient or other personnel are available for holding the programming head over the IMD. It has been recognized that distance telemetry systems that would allow telemetric communication between an IMD and external programmer or monitor over a distance of several meters, without the use of a programming head, is desirable. Telemetry sessions could occur while a patient is active, for example performing an exercise test or going about normal daily activities. A clinician performing tests or reprogramming the IMD could be freed of the task of maintaining the position of the programming head, making it easier for him/her to perform other tasks.
A distance or long-range telemetry system is generally disclosed in U.S. Pat. No. 6,482,154 issued to Haubrich et al. A distance telemetry system will transmit using high frequency RF signals, for example using UHF signals, between an antenna in a receiving mode and an antenna in a transmitting mode. Such systems utilize an IMD RF telemetry antenna that is located outside the IMD housing. The antenna is coupled to IMD circuitry, enclosed within the IMD housing, through an antenna feedthrough. Since the antenna feedthrough needs to conduct the telemetry RF signals into the IMD during telemetry sessions, the antenna feedthrough does not significantly filter the desired high frequency signals.
Cross-talk could occur between the unfiltered antenna feedthrough and any filtered feedthroughs included in the IMD. Such cross-talk would interfere with normal device sensing of signals on the filtered feedthroughs, potentially leading to inappropriate function of the IMD. In order to minimize EMI coupling that could occur across an unfiltered feedthrough and any filtered feedthroughs inside the IMD, a separate unfiltered feedthrough could be provided along the IMD housing, at a different location than a filtered feedthrough array. However, providing separate filtered and unfiltered feedthrough assemblies would likely increase IMD manufacturing complexity and/or cost. As IMD technology has advanced, the physical size of IMDs has decreased and the number of electrodes and sensors used with some IMDs has increased. As such, the physical space available for providing a separate unfiltered feedthrough is limited.